How to bake a cavity map from hi-poly to low-poly?
This is a "symptom" of a need for 'ray distance' :



The bake casts rays around the meshes.
If parts of the mesh you want to bake to are too far from the highpoly mesh, the bake will miss them.

Hypothesis : as you use here an emission material that can involve problems for parts of the lowpoly that are inside (not lighted).